drm/i915: less magic for stolen preallocated objects w/o gtt offset

A magic -1 is a obscure, especially since it's actually passed as an
unsigned, so depends upon the magic sign extension rules in C. This has
been added in

commit 3727d55e
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Wed May 8 10:45:14 2013 -0700

    drm/i915: allow stolen, pre-allocated objects to avoid GTT allocation v2

Use a proper #define instead. Spotted while reviewing Ben's
drm_mm_create_block changes.

v2: Cast the constant to u32 since otherwise we again have a type
mismatch. Suggested by Chris Wilson.

Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>